Jet drive not best for auxiliary sailboats
Recently, I received a letter from a reader asking about using a jet drive for a sailboat in the place of a propeller. While initially an intriguing prospect, jet drives have some disadvantages when used in this application.
Jet drives have been proposed for auxiliary propulsion for sailboats on and off since the 1930s, perhaps earlier. The chief advantage would be minimum drag under sail; no prop, no strut, no shaft. Some racers have even tried a closure flap over the flush inlet hole. This leaves an absolutely smooth hull when under sail. There is, though, no special advantage with regard to engine placement; the engine and drive shaft have to be located to connect to the impeller as directly as possible to minimize power losses. Using a pump that is separate from the engine wastes too much horsepower.
Unfortunately, the laws of physics make jet drive extremely inefficient for boats that operate at less than 25 knots, and that includes all sailboats. This is because, for a given horsepower, larger prop diameters are more efficient at transmitting power into the water as useful thrust.
The prop (or jet) pushes a boat forward by forcing a stream of water aft. The amount of force is a function of Newton's second law: force = mass x acceleration. Now, if one has a large-diameter prop, it is accelerating a large mass of water so the acceleration can be slower to generate a given thrust. Go to a smaller diameter, and less water's being forced aft, sofor the same thrustthe water has to be accelerated faster. The problem is that the prop (or jet impeller or pump) has only a brief instant to actually work on the water. For this reason, it is far more efficient to accelerate a large amount of water (large-diameter prop) slowly than a small amount of water quickly.
Jet thrust is, very roughly, equivalent to prop thrust of the same diameter. For an 18,000-pound, 28.75-foot waterline, steel 36-footer, the thrust required to drive her at hull speed (7.3 knots) happens to be approximately 1,000 pounds. This could be delivered with a 38-hp diesel driving a 21-inch-diameter three-bladed prop at 1,000 rpm. For many sailboats, however, there's really only room for about a 14-inch three-bladed prop. A typical small jet drive would be about four inches in diameter. But let's take a look at a more efficient six-inch- diameter unit (see box no. 1). One can see that efficiency, thrust, and speed go way down with decreasing diameter. Static thrust (bollard pull) is almost the same proportionately as thrust at speed in this case. What does this mean? It means that not only will one go much slower with the jet drive (with the same horsepower), but one will also have far less thrust to accelerate a boat into a head sea, or to get a quick response when going into reverse.
Of course, one can get the same thrust with a jet drive or with small- diameter props, but to make up for the lower efficiency, one has to substantially increase horsepower (see box no. 2).
Since fuel consumption and engine cost are proportional to horsepower, one will be paying a lot more for both engine and fuel for a jet drive rather than a larger-diameter prop.
Note that using a remotely-driven pump to power a jet drive will decrease efficiency still further due to mechanical losses. Driving the pump via hydraulics means there has to be a pump on the engine and a hydraulic motor at the impeller or jet pump. The hydraulic pump and motor are each about 85% efficient. Combined, that's a total drive efficiency of a bare 72%. By contrast direct-drive delivers about 95% of engine power. If one used a remote-pump jet-drive installation one would have to multiply the 22% efficiency by 72% to get a truly pitiful 16% efficiency. Instead of the already large 85-hp jet-drive engine, a 118-hp unit would be needed.
Interestingly, for powerboats that operate continuously at 20 to 30 knots, jets are only slightly less efficient, and they offer specific shoal-draft and low-speed handling advantages. As continuous operating speeds increase to 35 knots-plus, jets become more efficient than props. This holds true up to speeds of about 90 knots, where surface propellers are better.
Dave Gerr is a naval architect based in New York City.